This invention relates to motors for use in watchmaking, and more particularly to a single-phase stepping motor for a timepiece, of the type having a single coil, a single magnetic circuit, and two rotors driving two mechanically uncoupled gear trains.
This type of motor is used whenever there is a need for gear trains which are mutually uncoupled mechanically and driven by different rotors, as in a watch having electronic time correction, for example. Such stepping motors form part of the prior art.
Japanese Laid-open Patent Application No. 58-9086, an abstract of which appears in Patent Abstracts of Japan, Vol. 7, No. 78, p-188, 1223, of Mar. 31, 1983, describes a motor including two mechanically uncoupled rotors actuated by a single coil. A drawback of this type of motor is having a relatively large surface due to the presence of two magnetic circuits in parallel. Moreover, it follows from the drawing that the rotors are of the bipolar type with radial magnetization. Such rotors have only two stable balance positions per rotation. In order to display the sixty seconds positions on a watch dial, therefore, it is necessary to have a transmission ratio of 30 between the shaft of the rotor and the arbor of the seconds-hand. This transmission ratio generally involves having two gear trains.
Another motor comprising a double rotor and intended for a timepiece movement is described in French Disclosed Application No. 2,475,247. The object to be achieved according to that application is essentially different from that of the present invention since there it is solely a matter of compensating for the axial mechanical reaction appearing during use of a motor comprising a single disk as a rotor. Furthermore, the two disks are fixed on a common shaft and rotate at the same speed.
For other applications, motors comprising a disk-shaped double rotor are used; in particular, U.S. Pat. No. 3,922,574 describes a larger motor intended for a compressor, a pump, or a blower and having two disk-shaped rotors disposed on either side of an annular stator winding. In this case, too, the two rotors rotate at the same speed, it being possible to apply different torques to them by using permanent magnets of different strengths on each of the disks.
Swiss Patent No. 384,067 proposes another application of such a motor including two disk-shaped rotors, each comprising a number of different poles and being disposed on a common shaft which can slide longitudinally so that the speed of rotation of the shaft can be selected by causing one or the other of the rotors to drive.